Vet. Res. 31 (2000) 147-148

Non-vaccinated subpopulations play a major role in reinfections during eradication programs based on regular vaccination against Aujeszky's disease virus

V.F. Ohlinger^{a, b}, S. Pesch^{a, b} and I. Hense<sup>b

a</sup>  bioScreen European Veterinary Disease Management Center GmbH, Mendelstr. 11, Build. Ll, 48149 Münster, Germany
^b  State Institute for Chemistry, Food and Animal Health, von-Esmarch-Str. 12, 48149 Münster, Germany

Abstract - In 1996, Aujeszky's disease virus (ADV) was efficiently eradicated from the northwestern part of Germany, especially using ADV-gE-deleted modified live vaccines. Residual field virus infected pigs that reacted positive to ADV-gE antibodies were eliminated from the farms. In December 1996, a series of reinfections occurred along a road. The first outbreak was recognised in a cow that died on a swine farrowing-to-finishing farm. Pigs on that farm were diagnosed to be seropositive to ADV-gE. Serological surveys around this first outbreak revealed 3 positive herds 20 days after the initial outbreak, then an additional 3 positive herds after 33 days, 5 positive herds after 38 days and 1 positive herd after 45 days. The outbreaks were detected in routinely vaccinated herds within a distance of about 4 km, indicating a high pressure of ADV. New pathotypes of ADV and variations in antigenicity were ruled out by serological and molecular biological tests. Therefore surveys were initiated to prove the efficiency of the vaccines as well as to control vaccination within the farms. 5262 sows in breeding herds, 882 piglets in offsite weaning systems and 2759 pigs in finisher units were tested for antibodies to ADV-gE-/ADV-gB ⁺ as a parameter for vaccine induced antibodies. 2.23% of the sows, 12.47% of the offsite weaned piglets and 19.07% of the finishers were shown to react negatively to ADV vaccine induced antibodies, indicating no protective immunity to ADV. Based on these results, incomplete immunity was detected in 2% of the breeding units, 20.75% of the offsite nurseries and 23.40% of the finisher units. To prove the efficiency of vaccination against ADV, the serological status within various herds was evaluated. In 98% of the farrowing-to-finishing herds, all sows and finishers that were tested according to the official sampling protocol reacted positively to ADV-gB antibodies. A few animals were, however, shown to be non-vaccinated in the rest of the herds. Seronegative animals were found in gilt populations and finisher units, indicating no or delayed vaccination of gilts and no vaccination of piglets for finishing. Similar results were obtained from offsite weaning herds. 80% of the herds were found to be positive for vaccine antibodies. 14-100% of the animals that reacted negatively to vaccine antibodies were found in the rest of the nurseries. Whereas the official vaccination protocol was shown to be successful in the majority of the herds, ADV-seronegative pig populations were only shown to be present in a small number of herds. These populations were highly susceptible to ADV infections, leading to large amounts of virus spreading. ADV could be introduced into such herds via single seroreactors to ADV-gE (fieldvirus). One farm was shown to seroconvert to ADV-gE up to 84.2% within 7 weeks. However veterinarians and farmers did not believe any more in the lab results at the end of the eradication programs. A nested PCR to ADV-gE and ADV-gB was developed to verify EDTA-blood samples that previously reacted positively in serological assays for field virus and vaccine virus, respectively. During acute outbreaks, about 10% more sera reacted positively to ADV-gE in the nPCR than in the serological assay. Tonsils and EDTA blood samples of 106 single reactors, that had been eliminated from the herds were tested using nPCR to ADV-gE and ADV-gB to verify the serological results. 79.2% of the tonsils were found to be positive to ADV-gE and 76.4% positive to ADV-gB, respectively. 70.6% of the EDTA samples reacted positively to ADV-gE and 65.7% to ADV-gB, respectively. No PCR positive results were obtained from the 98 seronegative samples. Thus nPCR has been shown to be an efficient tool to verify seroreactors to ADV. In conclusion, about 76% of single seroreactors to ADV were shown not to be false positives. Therefore veterinarians and farmers could be highly convinced of the serological results by verification using nPCR. However, nPCR should not be used as an exclusion test in determining whether seroreactors are true or false positive to ADV-gE. Single seroreactors have to be eliminated from the farm immediately after confirmed positive diagnosis. Furthermore, existing vaccines were shown to be efficient in inducing anti-ADV immunity following vaccination schemes used during the eradication program. However, successful elimination of ADV field virus at the end of the eradication programs leads to a reduced discipline in vaccination to ADV with an increasing risk of reinfections. Thus vaccination has to be controlled by serological surveys to maintain ADV immunity.

Corresponding author: V.F. Ohlinger Tel.: (49) 251 980 1900; fax: (49) 251 980 1901;
    e-mail: ohlingerv@bioscreen-ms.de

© INRA, EDP Sciences 2000